Hi gents.

reading through a lot of topics in here at the moment trying to gather as much knowledge to be able to build my first rx7. The car was fully disassembled when I Bought it, so the journey is long.

well it is going to be a half bridge ported 12A with a modded Nikki. BUT I just had a look at the intake manifold and saw the primaries and secondaries are linked together in the manifold? I can’t really see this being optimal with a half bridge ported engine?

can anyone tell me if I should leave like this or what to do ? My thought was to connect the primaries with each other only.

 

I would not close the channels. If you do close the channels, bridge port the primaries, not the secondaries.

 

Oh dang! I saw a thread in here where it was discussed that these channels doesn’t work well with half bridge, and to my mind it make sense.

by bridgeporting the secondaries and street port the primaries it would make the bottom end street able and when it is needed the secondaries could open for max top end power with the bridge port, but only if the channel are closed (maybe channel the primaries together )

would you care to explain why you suggest the opposite ?

 

The bottom end is totally streetable even with a full bridge or peripheral port. Way more power at any engine speed over about 1500-2000 than a stock or street port. What is not streetable is idle or low load. Vacuum in the intake manifold pulls exhaust gases up into the intake manifold during the overlap period, causing what Mazda calls unstable combustion.

If you bridge ported the secondaries, and kept the secondaries isolated from the primaries, the secondary runners would see vacuum all the time (it is a little more complex than this but that is the gist) and it would run like crap all the time until you opened the secondaries up.

I used to have a half bridge Turbo II engine, bridge ported on the secondaries. I modified the stock throttle body so that all idle air went through the secondary runners, low speed drivability was massively improved.
A full bridge with an ITB type manifold drives better.

I would not bother with porting if you are using the stock manifold. The manifold and carburetor are the power restriction even on a stock engine. Plus, a bridge port will not idle with a Nikki unless you open the throttle plates so far that the transition slots are uncovered, and you will have no idle fueling control. The carburetor is simply far too small.

 

Nice! Thanks for the good explanation. I would definitely need to read up on this then! A full bridge would be nice, but doesn’t that require a 1 piece apex seal? I ordered original 2 piece. I thought a half bridge was a good compromise for high end street ?

I did now the Nikki was a restriction, that could easily be changed later on. I didn’t know the stock manifold would be an issue, but since these two can easy be swapped later on I would definitely still go with the bridge port, to save a rebuild of the engine later on again. I know myself enough to know that I would regret not doing it 🙂

So… If I keep my plan, but change to bridge on primaries and street port on secondaries, that would be the best compromise ?

 

To be honest, the drivability difference between half and full bridge is so incredibly minor that there is no point to not doing a full bridge I had three half bridge engines before I did a full bridge in 2012 and when I finally did that engine, I was blown away at how much different it wasn't.


This is with fuel injection. There's no way I could get drivability this good with a carburetor. I have not tried any kind of bridge port with a carb but I have had a peripheral port 12A and ran it carbureted and injected. Fuel injection was by far a better experience. Much better drivability, much less exhaust stink.

You can use two piece seals if you do not relieve the rotor housing. The engine in my car right now has two piece seals. Only the end ports are relieved, and the apex seals are installed backwards in the rear rotor. That said if you are using ferrous seals and not carbon aluminum, I would not exceed 8000rpm under any circumstances.

 

maybe just a street port would be a better option? stock carb and intake will be a better match, and it will drive just fine.

for the record i've been able to get the P Ports to drive really nicely with a carb, way better than expected. you do need to take the (long) time and tune everything though.

 

Thats a really great running car in the film Peejay! Driveability looks perfect! I agree this is near to impossible to get with a carb car. Injection would be awesome to convert to, however the laws in Denmark is crazy about mods on your car, so a stock carb (look at least) would be the best choice for me at the moment.

Yes, streetport with my current setup would proberly be the correct choice, BUT I'm pretty sure I would regret that choice pretty fast and than have to tear down the engine again Therefore I will do a bridge and then hopefully change the intake at some point down the road.

So maybe Full bridge with only one side housing relieved (secondaries), 2 piece original apex seals(Maybe I would regret that choice also). Then keep the RPM under 8k.

When you do a bridgeport, do you also open up the stock port to a streetport sizes ? I guess it depends on the setup.

 

in California we have an emissions check every two years, and its really common for people to have two setups, one that is legal and the other one they run the rest of the time. the intake is ~6 bolts, so it swaps really quickly.
its turned into a whole thing here just because we have to have California specific parts, but nobody wants to make or sell them here, so stuff that is legal is hard to get and expensive.

part 2, is that my P port has a stock air filter housing on top of the weber, and it basically fools everyone. they ask if its ported... stock air cleaner will limit you to ~150hp so a bridge will be a little limited
if i end up keeping it (it was a surprisingly difficult way to keep an air filter) the thing to do would be to add a second snorkel on it

 

My first bridge port was a heavily street ported Turbo II engine that I'd cut the eyebrows into.

You can bridge port a street port (as long as you don't move the opening line when you do the street port) but you can't undo a bridge.

Given your recommendations I strongly recommend taking his advice and doing a street port. Bridge ports do not tolerate intake vacuum or restrictive exhaust, and having a carburetor that small will make for the worst of all worlds.

My intake setup is a Holley 750cfm carb set up as a throttle body. With the Racing Beat manifold it is essentially a 4x42mm ITB setup. Even THEN it is a little small. The Nikki has something like 26-28 and 30-32mm throttle plates, and the throttle shaft takes up a much larger percentage of that (8mm diameter by recollection)

 

Damn! I really want the nice brab in idle and I would be really annoyed down the line if the port choice restricted me, let’s say I do injection at some point.

i also only have a bridge port template, let’s say I don’t cut the bridge, I guess the main port porting is quite conservative here ? Anyone got any suggestion to bigger main ports ?

 

I know we had this discussion on the forum before regarding the Scoot 13B LIM that has the secondary ports filled/blocked, runs on the primary ports only for low throttle/load, but runs off peripheral ports for high throttle/load using the FD UIM and 2-stage throttle body.

How does it run ok with PP blocked by a secondary throttle blade system against the statements here that it shouldn’t?

http://home.att.ne.jp/orange/NetWorks/mani4.htm
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they don't show it idling much, but there is enough adjustment in the throttle body to do a lot. the FD throttle body is like a carburetor it just doesn't have float bowls.
its one of the parts where you wonder how they made money on it

 

I don’t want to clutter up this thread on that other than to have asked the question. Possibly there are some variations to how it plays out on a carb vs EFI. I have zip on knowledge/experience when it comes to a rotary engine with a carb.

and there’s a semi PP variation currently moving forward here on the forum as well

https://www.rx7club.com/build-thread.../#post12400436
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he says it works well, never thought to ask about the idle, lol

PS i have an article where they have pics of the Scoot cars engine, and when i find it, i will post em up.
instead of dowels or studs they used the 20B bolt, which is bigger, and i think its got a roller bearing crank, but i don't remember if its an added center bearing or not